LEDs are a desirable choice of light source in part because of their relatively small size, low power/current requirements, rapid response time, long life, robust packaging, variety of available output wavelengths, and compatibility with modern circuit construction. These characteristics may help explain their widespread use over the past few decades in a multitude of different end use applications. Improvements to LEDs continue to be made in the areas of efficiency, brightness, and output wavelength, further enlarging the scope of potential end-use applications.
Recently, LEDs have begun to be used in illumination units for backlighting purposes in LCD television devices, as well as other types of lighting, signage, and display systems. For most lighting applications, it is necessary to have a plurality of LEDs to supply the required light intensity. Because of their relatively small size, a plurality of LEDs can be assembled in arrays having small dimensions and a high luminance or irradiance particularly if unpackaged or bare die LEDs are used.
It is possible to achieve an increase in the light density of an array of LEDs by increasing the packing density of the individual LEDs within the array. An increase in packing density can be achieved by increasing the number of LEDs within the array without increasing the space occupied by the array, or by maintaining the number of LEDs within the array and decreasing the array dimensions. However, tightly packing large numbers of LEDs in an array is a long term reliability concern since local heating, even with a globally efficient thermal conduction mechanism, can reduce the lifespan of the LEDs. Therefore, dissipating the heat generated by the array of LEDs becomes more important as the packing density of the LEDs increases.
In other applications, even those without high packing densities, the driving voltages/currents, size and brightness of LED dies are increasing, leading to increases in local temperatures around the LED dies. Consequently, there is a need for better heat dissipation at the location of each LED die, as well as across the array.
Conventional LED mounting techniques use packages like that illustrated in U.S. Patent Application Publication 2001/0001207A1 (Shimizu et al.), that are unable to quickly transport the heat generated in the LED away from the LED. As a consequence, performance of the device is limited. More recently, thermally enhanced packages have become available, in which LEDs are mounted and wired on electrically insulating but thermally conductive substrates such as ceramics, or with arrays of thermally conductive vias (e.g., U.S. Patent Application Publication 2003/0001488A1 (Sundahl)), or use a lead frame to electrically contact a die attached to a thermally conductive and electrically conductive thermal transport medium (e.g., U.S. Patent Application Publication 2002/0113244A1 (Barnett et al.)). An illumination assembly having improved thermal properties is disclosed in U.S. Patent Application Publication 2005/0116235A1 (Schultz et al.), in which an illumination assembly includes a plurality of LED dies disposed on a substrate having an electrically insulative layer on a first side of the substrate and an electrically conductive layer on a second side of the substrate. Each LED die is disposed in a via extending through the electrically insulative layer on the first side of the substrate to the electrically conductive layer on the second side of the substrate, and each LED die is thermally and electrically connected through the via to the electrically conductive layer. The electrically conductive layer is patterned to define a plurality of electrically isolated heat spreading elements which are in turn disposed adjacent a heat dissipation assembly.
Although the more recent approaches improve the thermal properties of LED arrays, there remains a continuing need for improved thermal properties, lower cost and simpler fabrication processes.